The automotive industry has placed increased emphasis on extending the lifetime of vehicle bodies, i.e., reducing the rusting of automobile sheet metal and the like, by use of various coating compositions designed to improve both corrosion resistance and chip resistance. Among areas on automobile and truck bodies particularly susceptible to chipping are wheel wells and lower portions of the bodies such as rocker panels and hoods. These areas can be subjected to the abrasive chipping action of road dirt and debris. For example, sand and gravel can strike these areas with a considerable impact velocity and result in aesthetically unpleasing stone chipping of automotive topcoats. Eventually such chipping can contribute to rusting. Thus, considerable efforts have been directed to developing coating compositions, e.g., primer compositions, having improved chip resistance as well as good corrosion resistance. A chip resistant primer composition should also have good intercoat adhesion, since such a composition is generally placed between an electrodeposited primer layer directly upon the metal and an outer topcoat layer. In areas such as wheel wells and underbodies, a chip resistant primer coating may not be topcoated and, therefore, should have resistance to weathering. Despite previous industry efforts, a need for effective chip resistant automotive coatings remains.
An additional concern facing the automotive industry is meeting governmental guidelines on volatile organic content (VOC) of automotive coatings. While there are numerous organic solvent based coating compositions useful as chip resistant primers (see, e.g., U.S. Pat. Nos. 4,581,424; 4,602,053; 4,608,313; and 4,614,683), these coating compositions each contribute to the VOC problem. Powder coating compositions can be especially useful in meeting VOC guidelines since they generally reduce or eliminate the use of organic solvents. However, few powder coating compositions have been described as chip resistant automotive primers (see, e.g., U.S. Pat. No. 4,251,426). Thus, a method of providing chip resistance by application of a powder coating composition would be especially beneficial to the automotive industry.